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Greenhouse gases (GHGs) trap heat within the earth's atmosphere. Although most GHGs occur naturally and help keep the earth hospitable to life, they are also generated by human activities. CO2 emissions account for more than 80 percent of the U.S. GHG emissions. These emissions are contributing to changes in the planet'sclimate that could lead to harmful effects, such as sea-level rise and changes in global hydrological patterns. Although the United States makes up 4.4 percent of the world'spopulation, it emits about 16 percent of carbon emissions from fossil fuel combustion.
Sources of Greenhouse Gases (2013)
Source: U.S. Environmental Protection Agency, U.S. Greenhouse Gas Inventory Report: 1990-2013, https://www3.epa.gov/climatechange/ghgemissions/usinventoryreport.html
Greenhouse gas emissions from transportation primarily come from burning fossil fuel for our cars, trucks, ships, trains, and planes. Overall, transportation contributes 27 percent of national GHG emissions.
Source: Boden, T.A., Marland, G., and Andres, R.J. (2015). National CO2 Emissions from Fossil-Fuel Burning, Cement Manufacture, and Gas Flaring: 1751-2011, Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, U.S. Department of Energy, doi 10.3334/CDIAC/00001_V2015. http://cdiac.ornl.gov/trends/emis/tre_coun.html
In contrast to most criteria pollutants, emissions of GHGs have been rising from most sectors. From 1990 to 2013, carbon emissions from transportation grew by 16 percent.
Change in Carbon Emissions (1990–2013)
Source: U.S. Environmental Protection Agency, U.S. Greenhouse Gas Inventory Report: 1990-2013, https://www3.epa.gov/climatechange/Downloads/ghgemissions/US-GHG-Inventory-2015-Main-Text.pdf
Highway vehicles are the largest users of transportation energy, accounting for 82 percent of the total.
Transportation Energy Use by Mode (2012)
Source: U.S. Energy Information Administration, Annual Energy Outlook 2015, Table A7–Transportation Sector Key Indicators and Delivered Energy Consumption, April 2015, http://www.eia.gov/forecasts/aeo/data/browser/.
The amount of energy used and its intensity is an indicator for the amount of GHGs produced. Per passenger mile, transit and passenger rail commuter transportation modes are less energy intense than light-duty passenger vehicles. However, light-duty trucks are more energy intense. For freight transport, heavy-duty trucks are considerably more energy intense than rail on a ton-mile basis.
Transportation Energy Intensity (2012)
Source: Bureau of Transportation Statistics, National Transportation Statistics, Table 4-20, 4-25: Energy Intensity of Passenger Modes, April 2015,
Transportation Energy Data Book: Edition 33, ORNL-6990, Oak Ridge National Laboratory, July 2014, http://cta.ornl.gov/data/tedb34/Spreadsheets/Table2_17.xls